A radar absorbing material (RAM) is a material which absorbs electromagnetic energy rather than reflecting it. Radar absorbing materials have obvious military applications, such as making aircraft, missiles and other equipment substantially less visible to radar. Radar absorbing materials also have commercial applications such as decreasing ghosting effects and unwanted clutter from structures interfering with airport radar systems and electromagnetic interference (EMI) control efforts on maritime vessels. Such commercial materials are available from companies such as the Plessey Company. Standard practice usually includes covering these objects with commercially available paste-on RAM materials. These materials can be difficult to apply and do not usually exhibit high-performance, multi-frequency radar absorption.
Commercially available materials typically include a single species of conductive filament with a specific length, diameter, conductivity and volume fraction within a dielectric binder to produce an absorber of limited bandwidth. Durability is also a major concern because commercially available RAM materials sometimes contain carbonyl iron powder (CIP) which oxidizes readily, if it is not totally encapsulated in an environmentally resistant binder.
A representative example of a prior art radar absorbing material is described in U.S. Pat. No. 3,599,210, entitled "Radar Absorptive Coating". U.S. Pat. No. 3,599,210 discloses a radar absorptive coating having conductive fibers cut to a length of one-half wavelength of the anticipated radar frequency. The fibers which may be formed of graphite are randomly dispersed in a lossy dielectric resinous binder. When radar signals impinge on the coating, the fibers act as tuned resonating dipoles for the particular radar frequency used, and the electromagnetic energy is dissipated in the lossy material.
Accordingly, there is a need for a more durable radar absorbing material having multi-frequency absorbing capabilities which can overcome the shortcomings of the known and commercially available radar absorbing materials.